If you are trying to train your Pokémon's Effort Values, Vitamins are a handy tool to utilise. Most vitamins add 10 Effort Values into a particular stat. There is a limit to how much you can do this, with the recent limit being that the Vitamins only add to the first 100 Effort Values in the stat. All vitamins are one-use only and they all slightly boost the Pokémon's happiness.